Driven shielding capacitive proximity sensor

ABSTRACT

A capacitive proximity sensing element, backed by a reflector driven at the same voltage as and in phase with the sensor, is used to reflect the field lines away from a grounded robot arm towards an intruding object, thus dramatically increasing the sensor&#39;s range and sensitivity.

ORIGIN OF THE INVENTION

This invention was made by employees of the U.S. Government andaccordingly may be manufactured and used by or for the Government forgovernmental purposes without the payment of any royalties thereon ortherefor.

.Iadd.This application is a continuation of application Ser. No.08/031,247, filed Mar. 11, 1993 now abandoned, which is a Reissue ofapplication Ser. No. 07/710,845 filed Jun. 6, 1991 now U.S. Pat. No.5,166,679. .Iaddend.

FIELD OF THE INVENTION

The present invention relates generally to proximity sensor apparatusand more particularly to a safety sensor in which a machine such as arobot senses the proximity to objects including humans at a rangesufficient to prevent collisions.

BACKGROUND OF THE INVENTION

Capacitive sensors used for proximity sensing are generally known;however, such sensors do not adequately control stray capacitance andconsequently do not perform with an adequate range and sensitivity formany applications. To overcome these limitations, the sensors are oftenmounted at substantial distances from the machinery, thus reducing theireffectiveness.

The purpose of the present invention is to provide a proximity sensorfor a robot arm that will sense intruding objects so that the robotcontroller can prevent the robot from colliding with objects in space,particularly a human being. This sensor must be able to functionreliably in the extreme environment of outer space and operates so asnot to disturb or be disturbed by neighboring instruments. It must besimple, compact and incidental to the robot design and be able to detectobjects at ranges in excess of 12 inches so that the robot hassufficient time to react. In the past a capacitive sensor typically wasmounted in a stand-off relationship from the grounded robot arm aconsiderable distance, typically 1 in. from the outer surface of thearm. This tends not only to disfigure the robot arm, but causes it to bebulkier than necessary. It also makes cross talk between the sensorelements more pronounced and tends to impede the flow of heat from therobot arm to outer space.

Although the driven shield technique is also known in conjunction withcapacitive sensors to prevent stray capacitance in lead wires and toincrease input impedance, it has also been utilized in connection withtactile sensors where pressure is exerted against one or both plates ofa capacitor.

SUMMARY

It is an object of the present invention, therefore, to provide animprovement in capacitive sensors.

It is a further object of the invention to provide an improvement incapacitive proximity sensors which significantly increase their rangeand sensitivity.

It is still another object of the invention to provide an improvedcapacitive sensor for proximity sensing insofar as it relates to arobot's ability to prevent collisions with objects coming intorelatively close proximity thereto.

Briefly, the foregoing and other objects are achieved by a robotic armproximity sensing skin which includes a capacitive sensing element,backed by a reflector driven at the same voltage as and in phase withthe sensing element. The reflector is used to reflect the electric fieldlines of the sensor capacitor away from the grounded robot arm towardsan intruding object. The sensor comprises a first thin sheet ofconductive material driven by an electronic circuit and forms oneelectrode of a capacitor, the second electrode of which is provided bythe intruding object and other nearby objects. A second thin sheet ofconductive material considerably wider than the sheet forming the sensoris inserted between the sensor and the grounded arm of the robot. Thesensor is thus shielded from the nearby ground such that the capacitancebetween it and the ground is substantially reduced, if not eliminated.The shield is driven through a voltage follower circuit by the samesignal that is coupled to an oscillator circuit whose output isinversely proportional to the capacitance of the sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of the invention will be more readilyunderstood when considered together with the .[.accompany.]..Iadd.accompanying .Iaddend.drawings in which:

FIG. 1 is a diagram generally illustrative of the present invention;

FIG. 2A is a diagram illustrative of the electrical field associatedwith a proximity sensor in accordance with known prior art;

FIG. 2B is a diagram illustrating the electric field distribution of aproximity sensor in accordance with the present invention; and

FIG. 3 is an electrical schematic diagram illustrative of the preferredembodiment of the invention wherein a driven shielding technique extendsthe range and sensitivity of a capacitive proximity sensor.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings and more particularly to FIG. 1, referencenumeral 10 denotes a robot having a base 12 which is shown beinggrounded via a support surface 14 and having an articulated armstructure 16 including a capacitive type proximity sensor 18 which isused to sense the presence of objects 20, such as human beings, whichcome into close proximity so that the robot can take appropriatemeasures to avoid collision with the object. This type of proximitysensing is essential not only in industry, but one very important use ofthis type of apparatus is to prevent a robot operating in space fromcolliding with a human being working in conjunction with or separatefrom the robot. The diagram of FIG. 1 also discloses an element 22beneath .Iadd.and spaced apart in substantially overlapping parallelfashion from .Iaddend.the sensor 18 and will be considered subsequentlysince it goes to the heart of this invention.

Typical prior art proximity sensing of an object 20 is known to involvethe .Iadd.use .Iaddend.of a capacitive sensor 18 which is mountedexternally of the robot arm 16 as shown in FIG. 2A. The stand-offdistance for example, is in the order of 1 in. The electric fieldassociated with this type of proximity sensor is furthermore shown bythe arrows emanating from the sensor 18 to ground via the skin of therobot arm 16 as well as back to the object 20 being sensed. Also, thereis an electric field from the object 20 to the grounded arm 16. Such aconfiguration has resulted in a sensitivity which is unacceptable forspace robotic applications where detection ranges in excess of 1 foot isrequired so that the robot 10, for example, can be deactivated orcommanded to take some type of evasive action in order to prevent acollision with the object 20.

The sensor 18 is essentially a thin sheet of conductive material drivenby an electronic circuit and acts as one electrode of a capacitor 19,with the second electrode being provided by the intruding object 20 asshown in FIG. 1, for example. The capacitor 19 controls the frequency ofan oscillator, not shown, so that when an object for some reason oranother intrudes, the output frequency of the oscillator changes. Thegrounded objects in the vicinity of the conductor sheet 18 and the leadwires, not shown, between the circuit and the sheet 18 create a largefixed parasitic capacitance which reduces sensor sensitivity to theintruding object 20.

To increase the sensitivity of the proximity sensor by reducing theparasitic capacitance exhibited by the electric field shown in FIG. 2A,the present invention provides for the insertion of a second relativelythin sheet 22 of conductive material, which is substantially wider thanthe sensor 18, .Iadd.spaced apart in substantially overlapping parallelfashion .Iaddend.between the sensor 18 and the grounded robot arm 16.This generates an electric field distribution as shown in FIG. 2B whereit can be seen that the member 22 acts as a shield for the capacitivesensor 18 wherein the field concentration is centered between it and theobject 20, with little, if any, of the field returning directly toground.

In operation, the shield member 22 is driven at the same instantaneousvoltage as the capacitive sensor 18, but is not frequency sensitive tonearby objects as is the sensor. Thus the sensor 18 is shielded fromnearby ground, i.e. the robot arm 16, such that the capacitance betweenit and the arm is substantially reduced if not eliminated.

Referring now to FIG. 3, a relatively low frequency oscillator circuitoperating at, for example, 20 kHz is coupled to a circuit node 26 towhich the capacitive type proximity sensor 18 is connected for theconfiguration shown in FIG. 2B. The total capacitance 19 between thesensor 18 and the intruding object 20 is shown comprised of thecapacitance C_(sg) of the sensor 18 to ground.Iadd., .Iaddend.i.e. thearm 16, the series combination of the capacitance C_(so) from the sensor18 to the object 20 and the capacitance C_(og) of the object 20 toground. The composite of these three capacitances comprise a tuningcapacitance for an oscillator 24 which is configured from an operationalamplifier (op amp) 28, the sensor 18, a voltage divider comprised of twofixed resistors 30 and 32, and the series resistors 34 and 36 which actas feedback resistors between the output of the op amp 28 and negative(-) and positive (+) inputs, respectively. A voltage follower circuitcomprised of an operational amplifier 38 couples the instantaneousvoltage at the circuit node 26 to the shield 22.

Since a relatively low frequency is generated, a quasi-static case.[.obtains.]. .Iadd.exists.Iaddend.. Assuming that a momentary positivepotential exists at circuit node 26, it can be seen that electric fieldlines (FIG. 2B) emanating from the sensor 18 towards the object 20induce negative charges on the object's surface nearest the sensor. Thusthat surface can be considered one plate of a capacitor 19 and thesensor 18 the other. An ungrounded conductive object, on the other hand,is charged neutral so that an equal amount of positive charge will formon the surface away from the sensor so as to ensure that there is no netelectric charge on the conductor. Accordingly, the tuning capacitancefor the oscillator 24 at node 26 is:

    (C.sub.so ·C.sub.og /(C.sub.so +C.sub.og))+C.sub.sg =C.sub.19. (1)

Since the object 20 shown in FIG. 1 is grounded, and C_(og) is thereforeshorted, equation (1) reduces to:

    C.sub.19 =C.sub.sg +C.sub.so                               (2)

In examining equations (1) and (2), since detection of small changes inC₁₉ are desired, it becomes evident that the capacitance from the sensorto ground C_(sg) must be relatively small. Therefore, the shield orreflector element 22 operates to force the field lines from the sensor18 towards the object 20 as much as possible as shown in FIG. 2B.

Considering the case where the object 20 is not grounded, it is knownthat:

    C=Q/V                                                      (3)

It is also known that a good conductor must have the same potentialeverywhere on its surface. Therefore, the potential on the object 20will be that of its farthest point from the sensor 18. If the potentialon the sensor 18 is defined as V and the potential on the object as V₀,then the following relationships are obtained:

    Q.sub.i /V-V.sub.0 =C.sub.so and                           (4)

    Q.sub.i /V.sub.0 =C.sub.og                                 (5)

Where Q_(i) is the charge induced on each side of the object. It isapparent that an object with any dimension more than a few inches in anydirection forces the potential on the entire surface of the object to bevery low and as experimental evidence indicates, all objects aresubstantially grounded.

Thus where a capacitive sensing element 18 is backed by a reflector 22driven at the same voltage as and in phase with the sensor, the fieldlines will be reflected away from the grounded robot arm 16 towards theintruding object 20. This dramatically increases the range by amagnitude of at least 10, i.e. from 1 in. to 12 in., for example.Furthermore, with this technique, capacitive sensor(s) 18 can be mountedcloser to grounded surfaces without any penalty in performance andmodern circuit techniques employing flexible printed circuit boards canbe utilized to great advantage.

Having thus shown and described what is at present considered to be thepreferred embodiment of the invention, it should be noted that the samehas been made by way of illustration and not limitation. Accordingly,all modifications, alterations and changes coming within the spirit andscope of the invention are herein meant to be included.

We claim:
 1. A capacitive type proximity sensor having improved rangeand sensitivity between a machine and an intruding object in theimmediate vicinity of the machine, comprising:an outer electricalconductor on said machine forming one electrode of a sensor capacitor,.[.the.]. .Iadd.an .Iaddend.other electrode comprising said.Iadd.intruding .Iaddend.object, said outer .Iadd.electrical.Iaddend.conductor comprising a first thin sheet of conductive material;an intermediate electrical conductor located between said outer.Iadd.electrical .Iaddend.conductor and said machine and being of a sizesubstantially larger than said outer .Iadd.electrical .Iaddend.conductorto act as a shield for reducing the parasitic capacitance between saidouter .Iadd.electrical .Iaddend.conductor and said machine, saidintermediate .Iadd.electrical .Iaddend.conductor comprising a secondthin sheet of conductive material substantially wider than said.[.first.]. .Iadd.outer electrical .Iaddend.conductor; said outer andintermediate conductors attached to a surface on said machine .[.with nogap between the conductors.]. .Iadd.wherein said conductors are spacedapart in substantially overlapping parallel fashion, .Iaddend.and no gapbetween said surface and said intermediate conductor and .[.said outerand intermediate conductors coplanar with each other.]. and said surfaceof said machine, said surface of said machine acting as a groundplane.Iadd., with said intermediate electrical conductor and saidsurface of said machine spaced apart in substantially overlappingparallel fashion.Iaddend.; first circuit means for coupling in phase theinstantaneous voltage at said outer electrical conductor to saidintermediate electrical conductor; second circuit means coupled to saidouter .Iadd.electrical .Iaddend.conductor and being responsive to thecapacitance of said sensor capacitor for generating a control signal tosaid machine.
 2. The sensor of claim 1 wherein said first circuit meanscomprises a voltage follower circuit comprised of an operationalamplifier.
 3. The sensor of claim 2 wherein said second circuit meanscomprises a signal generator having an output signal indicative of thecapacitance of said sensor capacitor.
 4. The sensor of claim 3 whereinsaid signal generator comprises a variable frequency oscillator.
 5. Thesensor of claim 1 wherein said machine comprises a robotic apparatus. 6.The sensor of claim 5 wherein said robotic apparatus comprises a movablemember of said robotic apparatus.
 7. The sensor of claim 6 wherein saidmovable member comprises an articulated member.
 8. The sensor of claim 7wherein said articulated member comprises a robotic arm.